Variable magnification endoscope

ABSTRACT

An endoscope whose magnification may be continuously varied from zero to a microscopic magnification of 40x or more while providing full correction for aberrations at widely different magnifications. The objective comprises a positive lens group in a microscope objective configuration, and a front group which typically includes a negative lens and a physical stop in front of the positive lens group generally near the focal plane of the positive lens group. To achieve high magnification, the objective is moved away from the transfer optics, and placed in contact with the object to be viewed. In such a configuration, the positive lens group functions as a microscope objective while the negative lens group&#39;s contribution to the aberrations is small. At low magnification, the negative lens group cooperates with the positive lens group to provide a wide angle lens. Pupil stabilization is achieved by placing the physical stop so that when the endoscope is used in the microscope mode, the physical stop between the positive and negative lens groups is ineffective or marginally effective. This permits the maximum numerical aperture consistent with the physical diameter of the positive lens group. However, the physical stop between the positive and negative lens groups comes into play when the object plane moves away from the negative lens group. The field lens located at the image plane receives the marginal chief ray at a nearly constant small angle over the entire range of magnification. This allows image transfer to be achieved with conventional means such as alternating field and relay lenses.

This is a continuation of application Ser. No. 133,732, filed Mar. 25,1980 now abandoned.

FIELD OF THE INVENTION

The present invention relates generally to endoscopes, and morespecifically to variable magnification endoscopes.

BACKGROUND OF THE INVENTION

An endoscope is an optical instrument for viewing and examining theinterior of various body cavities, such as the lung, bladder, abdominalcabity or knee joint. Access to the interior body region may be via anatural body conduit or, in the case of body cavities not so accessible,by a small surgical incision. Broadly, the endoscope comprises a longthin tube, the distal (leading) end of which is inserted into thepatient. An eyepiece is mounted to the proximal end of the tube, andwithin the tube are housed an image-forming objective at the distal end,fiber optic or discrete transfer optical elements for transmitting theoptical image formed by the objective to the eyepiece outside the bodyfor viewing by the examining physician, and fiber optic elements fortransmitting light from outside the patient to the interior regionsunder examination for illumination thereof. The illumination fiberoptics typically occupy an annular region surrounding the objective andtransfer optical elements. When discrete transfer optics are used ratherthan fiber optics, the transfer optics typically include a front fieldlens which defines the image plane for all magnification conditions, andalternating relay and field lenses. The endoscope may be rigid orflexible, depending on its intended use.

The realm of endoscopic procedures includes, in addition to examination,the excision and removal of tissue. For example, it is a commontechnique to remove polyps and tumors by such techniques, thus avoidingopen surgery. This is accomplished by providing blades or hot wires atthe end of the endoscope for excision, and appropriate conduits forwithdrawal of the excised tissue.

It often happens that endoscopic exploration is performed in conjunctionwith a biopsy. In such a procedure, the surgeon explores the interior ofthe body cavity, and upon noticing a suspicious looking region having atumor, excises a small piece of tissue which is withdrawn for thebiopsy. The biopsy is a procedure involving sectioning and microscopicexamination in a pathology lab. The result of the biopsy is typicallymade available within a day or two, and if the results indicate amalignancy, the patient submits to surgery for removal of the tumor, orundergoes other appropriate treatment.

It is immediately apparent that while the use of the endoscopeeliminates open surgery for the performing of the biopsy, and furthercan even avoid open surgery for removal of the tumor, the presentprocedure involving an intermediate pathological examination requiresthat the patient be subjected to two surgical procedures, each of whichmay have to be accompanied by a general anesthetic. Furthermore, thebiopsy results may cause the surgeon to wish to perform further biopsyprocedures.

This difficulty could be ovecome if an endoscope capable of viewingmicroscopically as well as telescopically could be used. With such aninstrument, the examining physician could scan the interior region, andupon noticing a suspicious region, directly view in situ the singlecells to make a pathological determination during the course of a singleendoscopic procedure. Variable magnification endoscopes re known. Atypical technique for achieving this result is shown in U.S. Pat. Nos.3,608,998 and 4,076,018 and provides variable magnification elementsnear or in the eyepiece of the endoscope. Such an arrangement has theclear disadvantage that the resolution of the instrument is fixed oncethe front lens group (objective) is fixed. Thus, at the microscopicsetting, the increased magnification is likely not to be accompanied bycorrespondingly increased resolution. A further difficulty withproviding a variable magnification endoscope that allows truemicroscopic as well as telescopic examination arises from the greatdifficulty in correcting a lens for more than one set of conditions,unless a zoom lens having elements movable with respect to one anotheris used. While it is conceptually easy to visualize such a zoom lens atthe distal end of the endoscope, such a solution is highly impracticaldue to the fact that zoom lens designs dictate a complex mechanicalconfiguration and many lens elements. Since endoscope lens elementstypically have a diameter in the neighborhood of two to threemillimeters, a zoom lens is clearly impractical in the context of anendoscope.

U.S. Pat. No. 3,941,121 discloses an endoscope having an objective, theelements of which are fixed relative to one another, that is relativelymovable with respect to a fiber optics transfer system. However, at thehigh magnification necessary for microscopy, the objective disclosedtherein is incapable of producing the necessary image resolution. Morespecifically, it is impossible to obtain a sufficient numerical apertureat the level of correction necessary for performing pathologicalmicroscopy.

A further difficulty arises in the interaction between the objective andthe transfer system. The use of a fiber optics transfer system tends tobe accompanied by a loss in resolution. While this situation can beimproved somewhat through the use of exotic techniques such as vibratingthe input end of the fiber bundle or introducing aberrations which aresubsequently corrected, maximum resolution is still likely to beachieved through the use of discrete lenses (alternating field and relaylenses). However, when discrete transfer optics are used, the problem ofpupil coupling is aggravated. In particular, pupil coupling is criticalin order to provide evenness of illumination, but where the objectivemoves relative to the transfer optics, the exit pupil location varies somuch with large magnification changes that the image transfer to theeyepiece suffers unacceptable vignetting.

U.S. Pat. No. 4,111,529 discloses an endoscope objective for use in wideangle viewing, but the objective does not possess sufficient resolutionto render it suitable for microscopic viewing. Moreover, the numericalaperture is insufficient due to the stop positioning disclosed.

Therefore, while variable magnification endoscopes are known, andmicroscopic endoscopes are known, there is presented the need for anendoscope that provides a continuous variation of magnification over awide range while providing suitable corrections and pupil coupling undersuch widely varying conditions.

SUMMARY OF THE INVENTION

The present invention provides an endoscope whose magnification may becontinuously varied from zero to a microscopic magnification of 40× ormore at the objective while providing full correction for aberrations atwidely different magnifications.

The above results are achieved by moving the objective only with respectto the transfer optics. The objective comprises a positive lens group ina microscope objective configuration, and a front group which typicallyincludes a negative lens group (which may comprise a single element) anda physical stop in front of the positive lens group (between thepositive and negative lens groups), the stop being located generallynear the focal plane of the positive lens group.

To achieve high magnification, the objective is moved away from thetransfer optics, and placed in contact with the object to be viewed. Insuch a configuration, the outer surface of the negative lens groupfunctions as an object plane locator. The positive lens group functionsas a microscope objective while the negative lens group's contributionto the aberrations is small. At low magnification, the negative lensgroup cooperates with the positive lens group to provide a wide anglelens. In such a configuration, the negative lens group acts as a fieldexpander.

Although the wide range of operating conditions makes a normal computersolution for an optimum set of optical parameters ill-conditioned, thepresent design allows a convergent solution as follows. The positivelens group is initially designed as a high power microscope objectivewhose parameters are chosen to substantially fully correct aberrations.This is done without reference to the front group. The parameters forthe negative lens group and the stop are then chosen to optimize theobjective as a wide angle lens. The field aberrations of the objectivein the wide angle mode are typically most sensitive to the stop locationand size. The stop parameters (as well as those of the negative lensgroup), once established, may have a deleterious effect on theperformance in the microscope mode. The positive group is thenre-optimized as a microscope objective, leaving the parameters of thenegative group and the stop fixed, but taking them into account in theoverall optimization. This sequence may then be repeated to provide fullcorrection for two widely different magnifications. At intermediatemagnifications, the objective is surprisingly well corrected.

According to a further aspect of the present invention, pupilstablilization is achieved by placing the physical stop so that when theendoscope is used in the microscope mode, the physical stop between thepositive and negative lens groups is ineffective or marginallyeffective. This permits the maximum numerical aperture consistent withthe physical diameter of the positive lens group. Regardless of whetherthe pupil is at the stop or the positive group, at high magnification,the chief ray is nearly parallel to the optic axis since the distancebetween the objective and the image plane is large relative to thediameter of the lens. However, the physical stop between the positiveand negative lens groups comes into play when the object plane movesaway from the negative lens group. The location of the stop is chosen tobe near the focal plane of the positive lens group, thus providing asubstantially parallel chief ray for conditions of lower magnification.Thus, the field lens located at the image plane receives the marginalchief ray at a nearly constant small angle over the entire range ofmagnification. This allows image transfer to be achieved withconventional means such as alternating field and relay lenses.

For a further understanding of the nature and advantages of the presentinvention, reference should be made to the remaining portions of thisspecification and to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a simplified sectional oblique view of a variablemagnification endoscope according to the present invention, with theendoscope in a position for producing wide-angle viewing at lowmagnification;

FIG. 1B is a similar view of the endoscope in a position formicroscopic, large magnification viewing;

FIG. 2A is an optical schematic of the endoscope objective;

FIG. 2B shows a modified front element;

FIGS. 3A and 3B are ray diagrams showing image formation and pupildefinition in the microscope mode;

FIGS. 4A and 4B are ray diagrams showing image formation and pupildefinition in the wide-angle, telescopic mode;

FIG. 5 is a ray diagram illustrating the role of the intermediatephysical stop in the wide angle mode; and

FIG. 6 is a ray diagram showing image formation and pupil definition atan intermediate magnification setting;

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1A is a simplified, sectioned, oblique view of an endoscope 10according to the present invention. Endoscope 10 is an elongateinstrument characterized by a distal end 12 for insertion into apatient's internal body region and a proximal end 15 that remainsoutside the patient. Transverse dimensions are greatly exaggerated forclarity; it should be understood that endoscope 10 typically has alength of about 250 millimeters and an outer diameter of about 5millimeters.

The mechanical construction of the endoscope is defined primarily bythree coaxial thin-walled tubes 17, 20, and 22, of decreasing diameters,with outer tube 17 defining the outermost diameter of the endoscope.Outer tube 17 and middle tube 20 are fixed relative to one another andsized to define therebetween an annular region within which is located afiber optics lighting bundle 25 having an input end 27 near proximal end15 and an output end 30 generally at distal end 12. As will be describedbelow, at least a portion 31 of bundle 25 may be angled to terminate sothat the fibers therein are directed toward the central axis. Inner tube22 is movable relative to tubes 17 and 20, and is sized to slidesmoothly within the bore of middle tube 20. This relative slidingmovement provides variable magnification as will be described in detailbelow. In order to provide a seal between the interior of tube 20 andthe outside atmosphere, appropriate sealing means such as an O-ring seal32, is provided. A groove 33 in inner tube 22 and a ball plunger 34provide a positioning detent to facilitate relative positioning.

An objective lens system 35, to be described in detail below, is rigidlymounted within middle tube 20 at proximal end 12. The remaining opticalcomponents are rigidly mounted within inner tube 22 so that they maymove relative to objective 35 when inner tube 22 is moved relative toouter tube 17. In particular, the optical elements mounted within innertube 22 include a transfer system defined by a plurality of field andrelay lenses 40 and an eyepiece 45 at the end of inner tube 22 remotefrom objective 35. Lenses 40 are preferably rod lenses of a type wellknown in the endoscope art. Alternately, a fiber optics transfer systemwithin inner tube 22 may be used. An eyepiece ring 50 surrounds eyepiece45 and is rigidly mounted to inner tube 22. The image formed byobjective 35 must be located in a plane 52 fixed relative to thetransfer optics in order to provide proper eyepiece image formation.Thus, depending on the distance between objective 35 and the object tobe viewed, inner tube 22 must be moved in order to properly locate theimage formed by objective 35.

FIG. 1A shows endoscope 10 with inner tube 22 at a position of closestapproach to objective 35. FIG. 1B is a similar view to FIG. 1A exceptthat inner tube 22 is in a position farther removed from objective 35.As will be discussed below, this latter position provides highmagnification. Thus, from a physician's point of view, examinationproceeds as follows. Endoscope 10 is inserted into the patient while inthe position shown in FIG. 1A. This provides a wide-angle view thatpermits the physician to scan the interior of the region underexamination. When a closer examination of a particular spot is desired,the physician moves distal end 12 closer to the spot and withdraws innertube 22 by an amount necessary to provide a focused image in eyepiece45. When microscopic examination is required, the outer (front) surfaceof objective 35 is brought into proximity with the spot to be examined,and inner tube 22 is withdrawn to the position of FIG. 1B whichrepresents the maximum extension that has any practical usefulness. Inthe preferred embodiment, microscopic examination occurs with the frontsurface of objective 35 in actual contact with the spot underexamination.

FIG. 2A is an optical schematic of objective 35. Objective 35 comprisesa front group 58 and a magnifying positive lens group 62. Front group 58preferably comprises a negative element 60 and an aperture stop 65located between negative element 60 and positive group 62. Negativeelement 60 preferably has a convex or planar front surface 60a which isexposed at distal end 12 and is brought into contact with the object tobe microscopically examined. Positive lens group 62 comprises a triplethaving elements 70, 72 and 74 with the latter two forming a cementeddoublet. All the lens elements are centered on a common optic axis 76.

FIG. 2B illustrates an alternate form of front negative element 60,designated 60', modified to permit illumination of an object contactingthe front surface, designated 60a'. To accomplish this, peripheralregions of element 60' are cut away to define a frustoconical peripheralsurface 60b'. The ends of the fibers in bundle portion 31 terminate atsurface 60b' so that the light emanating from the fiber ends is directedforward toward the central region of surface 60a'.

As was described above, microscopic examination occurs with frontsurface 60a of negative lens 60 contacting the object, thus acting as anobject plane locator. The contribution of negative lens 60 to theaberrations is small, thus allowing the parameters of positive lensgroup 62 to be chosen to provide a substantially fully correctedmicroscope objective. Once front lens surface 60a is moved away from theobject, negative lens 60 acts as a field expander, and its effect onfield aberrations is no longer small. Thus, the parameters of negativelens 60 and aparture stop 65 may be chosen to correct the fieldaberrations of the entire objective at the position of lowmagnification.

The above description regarding the selection of the optical parametersis somewhat of an oversimplification, since the parameters of positivegroup 62 are affected by the insertion of negative element 60 andaperture stop 65. In particular, when these elements (especially stop65), are inserted, the performance of positive group 62 as a microscopeis typically degraded. However, re-optimization may be achieved, as forexample by adjusting the thicknesses of one or more of the tripletelements to effectively eliminate the effect of aperture stop 65 duringmicroscopic viewing. Once this is done, the front group may bere-adjusted to further optimize the objective for wide angle viewing.This process may be iterated to bring the aberrations to a desired levelof correction. Since the effect of the front group on microscopicviewing is small (once the stop effect is minimized or eliminated), theparameters for the front group and the positive group become generallydecoupled, and the iterative process is convergent. With objective 35thus corrected at its two extremes of magnification, it has been foundthat aberrations at intermediate magnifications are surprisingly small.A further constraint on the location and size of aperture stop 65 isimposed by pupil coupling considerations to be described below.

FIGS. 3A and 3B are ray diagrams showing the formation of an image 77 inimage plane 52 for an object 78 located at front surface 60a of negativelens 60 FIG. 3B is a fragmented enlargement of FIG. 3A. In thisposition, corresponding to FIG. 1B, the physical mounting of positivelens group 62 defines the stop location at a position 79. Given thelarge distance between image plane 52 and objective 35 in relation tothe diameter of inner tube 22, the marginal chief ray, designated 80, isat a small angle (about 1°) with respect to the optic axis. In thisposition, aperture stop 65 serves no function so that an image ofmaximum brightness occurs.

FIGS. 4A and 4B are ray diagrams showing the formation of an image 82 inimage plane 52 for telescopic viewing of an object 85 positioned in anobject plane 87 at a substantial distance from front surface 60a. Themaximally off-axis bundle is characterized by a chief ray 90 which isincident on the transfer optics at the same small angle as in FIGS. 3Aand 3B. This matching of the chief ray angle results from thepositioning of stop 65 proximate the focal plane of positive lens group62. To understand the significance of stop 65, reference should be hadto FIG. 5 which shows image formation as in FIG. 4B, except for theabsence of stop 65. Under these circumstances, the stop location isdefined by the positive lens mount, and thus the maximally off axisbundle is characterized by a chief ray 91 which is incident on thetransfer optics at a relatively large angle (say about 14°). This leadsto unacceptable non-uniformity of illumination over the entire field ofview if discrete transfer optical elements are used.

FIG. 6 is a ray diagram showing the formation of an image 92 in imageplane 52 for viewing of an object 95 in an object plane 97 at anintermediate distance from front surface 60a. For this position ofapproximate unit magnification, aperture stop 65 remains effective tostabilize the pupil so that the marginal chief ray enters the transferoptics at a small angle. As the magnification is increased, the aperturestop becomes less effective so that it is ineffective or only marginallyeffective at the microscopic position described above.

For purposes of illustration, the optical parameters of a particularembodiment will be set forth. The geometric parameters include the radiiand separation of all the element surfaces. In order to make thenomenclature uniform and transparent, and referring to FIG. 2A, surfacesproceeding back from front surface 60a will be numbered in numericalorder with surface 60a being designated for the present purpose assurface 101. In particular, negative lens element 60 is characterized byfront and rear surfaces 101 and 102, aperture stop 65 is located withina plane 103, positive singlet 70 has surfaces 104 and 105, doubletelement 72 has surfaces 106 and 107, and doublet 74 shares surface 107and has rearmost surface 108. The radius of a given surface 10i will bedenoted by r_(i) so that front surface 101 has radius r₁, whilerear-most surface 108 has radius r₈. Aperture stop has a diameter d₃.Additionally, the distance between a given pair of adjacent surfaces 10jand 10k will be designated t_(jk). Moreover, the index of refraction anddispersion factor of the medium between surfaces 10j and 10k will bedesignated n_(jk) and df_(jk). With this nomenclature established, theparameter for a specific embodiment are given in Table I. Dimensions arein millimeters.

    ______________________________________                                        r.sub.1 =                                                                            ∞                                                                       t.sub.12 = 0.250000                                                                        n.sub.12 = 1.792266                                                                       df.sub.12 = 0.064                             r.sub.2 =                                                                            1.798864                                                                      t.sub.23 = 0.300000                                                                        air                                                       d.sub.3 =                                                                            0.600000                                                                      t.sub.34 = 0.759514                                                                        air                                                       r.sub.4 =                                                                            -13.198478                                                                    t.sub.45 = 1.000000                                                                        n.sub.45 = 1.885793                                                                       df.sub.45 = 0.156                             r.sub.5 =                                                                            -1.798864                                                                     t.sub.56 = 0.100000                                                                        air                                                       r.sub.6 =                                                                            8.851848                                                                      t.sub.67 = 0.600000                                                                        n.sub.67 = 1.855035                                                                       df.sub.67 = 1.033                             r.sub.7 =                                                                            1.798864                                                                      t.sub.78 = 1.000000                                                                        n.sub.78 = 1.554398                                                                       df.sub.78 = 0.069                             r.sub.8 =                                                                            -3.077912                                                              ______________________________________                                    

The objective described above is color corrected for blue (4800A), green(5461A) and red (6438A) light with equal spectral weights. The resultingobjective has an effective focal length of 2.0665 mm and a back focus of85.5176 mm with a magnification of -39.969367 at a numerical aperture of0.5 in the microscopic mode. The front focus is located near (slightlybehind) front surface 101 to provide such high magnification. This isalso seen in FIG. 6 where the ray emerging parallel to the optical axiscrosses the axis just inside the negative front element.

In summary it can be seen that the present invention provides anendoscope objective that is substantially fully corrected for fieldaberrations over an extremely wide range of magnification. Moreoever,the design provides a stabilized pupil location when such is necessary,as when discrete transfer optical elements are used. For an endoscopeusing fiber optics transfer, the pupil stabilization requirement may berelaxed and parameters varied to provide other advantages such asgreater ease of fabrication and the like.

While the above provides a full and complete disclosure of the preferredembodiments of the invention, various modifications, alternateconstructions, and equivalents may be employed without departing fromthe true spirit and scope of the invention. For example, it should bepossible to make front element 60 flat on both of its surfaces, relyingon aperture stop 65 to optimize the objective as a wide-angle lens. Insuch a case, aperture stop 65 would typically be defined by a coating onthe rear surface of front element 60. Moreover, while a particularphysical stop location and size are shown, it should be understood thatthe aperture stop could in principle be located at other opticallyequivalent points. Therefore, the above description and illustrationsshould not be construed as limiting the scope of the invention which isdefined by the appended claims.

We claim:
 1. An endoscope having distal and proximal ends,comprising:optical transfer means for transferring at fixedmagnification an image formed at an image plane to said proximal end,said image plane being fixed relative to said optical transfer means; anegative lens at said distal end, said negative lens having a frontsurface; a positive lens between said negative lens and said imageplane; said positive lens and said negative lens being characterized byrespective first and second sets of optical parameters, each of saidsets of optical parameters including geometrical parameters, namelysurface radii and surface locations along an optical axis, and opticalmaterial parameters, namely refractive index and dispersion values;means for rigidly coupling said positive lens and said negative lens toone another while preventing relative movement therebetween to define anobjective; and slidable support means for permitting said objective tomove with respect to said image plane over a range along said opticalaxis, said range including a first position of maximum magnificationwherein an object proximate the front surface of said negative lens isimaged in said image plane, and a second position of relatively lowermagnification wherein an object at a substantial distance from saidfront surface of said negative lens is imaged in said image plane; saidfirst and second sets of optical parameters being chosen such that saidobjective in said first position operates as a substantially fullycorrected microscope objective and said objective in said secondposition operates as a wide angle lens having substantially fullycorrected field aberrations; the operation of said objective as asubstantially fully corrected microscope objective being largelydetermined by said first set of optical parameters with said negativelens contributing little to the aberrations and overall power of saidobjective; said second set of optical parameters being chosen to correctfield aberrations of said object when said objective operates as a wideangle lens.
 2. The invention of claim 1 wherein said positive lens ischaracterized by a focal plane located between said positive lens andthe front surface of said negative lens, and further comprising anaperture stop located at a position that is optically equivalent to aposition in the vicinity of said focal plane such that the marginalchief ray intersects said image plane in a direction approximatelyparallel to said optical axis for image formation in said secondposition of low magnification.
 3. An objective for placement at aleading end of an endoscope comprising:magnification means characterizedby an optic axis; and a front group including means defining a frontsurface; means for rigidly coupling said magnification means and saidfront group while preventing relative movement therebetween; said frontgroup and said magnification means being characterized by respectivefirst and second sets of optical parameters chosen to define a frontfocus of said objective which is located proximate said front surfacesuch that said objective operates as a substantially fully correctedmicroscope objective for magnification of an object located at aposition proximate said front surface, and said objective operates as awide angle lens having substantially fully corrected field aberrationsfor imaging of an object located a substantial distance in front of saidfront surface, each of said sets of optical parameters includinggeometrical parameters, namely surface radii and surface locations alongsaid optical axis, and optical material parameters, namely refractiveindex and dispersion values; the operation of said objective as asubstantially fully corrected microscope objective being largelydetermined by said fist set of optical parameters with said front groupcontributing little to the aberrations and overall power of saidobjective; said second set of optical parameters being chosen to correctfield aberrations of said objective when said objective operates as awide angle lens.
 4. The invention of claim 3 wherein said positionproximate said front surface is a position contacting said frontsurface.
 5. The invention of claim 3 wherein said magnification meanscomprises a positive lens group in a microscope objective configuration.6. The invention of claim 3 wherein said front group comprises anegative lens.
 7. The invention of claim 3 or 4 or 6 wherein said frontgroup includes stop means located such that the marginal chief ray isapproximately parallel to the optic axis for an object at a substantialdistance ahead of said front surface.
 8. The invention of claim 3 or 4wherein said front group comprises a negative lens and an aperture stoplocated physically between said magnification means and said negativelens.
 9. An endoscope having distal and proximal ends,comprising:optical transfer means for transferring at fixedmagnification an image formed at an image plane to said proximal end,said image plane being fixed relative to said optical transfer means;magnification means characterized by an optic axis; a front groupincluding means defining a front surface; means for rigidly couplingsaid magnification mens and said front group to one another whilepreventing relative movement therebetween to define an objective; andslidable support means for permitting said objective to move withrespect to said image plane over a range along said optical axis, saidrange including a first position of maximum magnification wherein anobject located at a position proximate said front surface is imaged insaid image plane, and a second position of relatively lowermagnification wherein an object at a substantial distance from saidfront surface is imaged in said image plane; said magnification meansand said front group being characterized by respective first and secondsets of optical parameters chosen such that said objective operates as asubstantially fully corrected microscope objective for said firstposition and as a wide angle lens that has substantially fully correctedfield aberrations for said second position, each of said sets of opticalparameters including geometrical parameters, namely surface radii andsurface locations along said optical axis, and optical materialparameters, namely refractive index and dispersion values; the operationof said objective as a substantially fully corrected microscopeobjective being largely determined by said first set of opticalparameters with said front group contributing little to the aberrationsand overall power of said objective; said second set of opticalparameters being chosen to correct field aberrations of said objectivewhen said objective operates as a wide angle lens.
 10. The invention ofclaim 9 wherein said position proximate said front surface is a positioncontacting said front surface.
 11. The invention of claim 9 wherein saidmagnification means comprises a positive lens group in a microscopeobjective configuration.
 12. The invention of claim 9 wherein said frontgroup comprises a negative lens.
 13. The invention of claim 9 or 10 or12 wherein said front group includes stop means located such that themarginal chief ray is approximately parallel to the optic axis for anobject at a substantial distance ahead of said front surface.
 14. Theinvention of claim 9 or 10 wherein said front group comprises a negativelens and an aperture stop located physically between said magnificationmeans and said negative lens.
 15. In an endoscope having distal andpoximal ends, optical transfer means for transferring at fixedmagnification an image formed at an image plane fixed relative to saidoptical transfer means to said proximal end, and an objective movablerelative to said image plane along an optic axis, said objective beingmovable over a range including first and second widely spaced positionsto permit normal microscopy at large magnification and normal endoscopyat relatively small magnification, the improvement wherein saidobjective comprises:a positive lens group in a microscope objectiveconfiguration; and a front group including a front element and anaperture stop located between said front element and said positive lensgroup; means for rigidly coupling said positive lens group and saidfront group while preventing relative movement therebetween; saidaperture stop being located so as to be largely ineffective when saidobjective is in said first position of large magnification and so as tomaintain the marginal chief ray approximately parallel to the optic axisfor an object at a substantial distance ahead of said front surface;said positive lens group and said front group being characterized byrespective first and second sets of optical parameters chosen such thatsaid objective operates as a substantially fully corrected microscopeobjective for said first position and as a wide angle lens that hassubstantially fully corrected field aberrations for said secondposition, each of said sets of optical parameters including geometricalparameters, namely surface radii and surface locations along saidoptical axis, and optical material parameters, namely refractive indexand dispersion values; the operation of said objective as asubstantially fully corrected microscope objective being largelydetermined by said first set of optical parameters with said front groupcontributing little to the aberrations and overall power of saidobjective; said second set of optical parameters being chosen to correctfield aberrations of said objective when said objective operates as awide angle lens.
 16. The invention of claim 13 wherein said opticalparameters provide generally fully corrected field aberrations atpositions intermediate said first and second positions.
 17. Theinvention of claim 15 wherein said front group comprises a negative lensand an aperture stop located physically between said magnification meansand said negative lens.
 18. The invention of claim 3 or 9 or 15 whereinsaid front group includes means defining a peripheral surface havingportions with normal axis at an acute angle with respect to said opticaxis to permit illumination of said object located at said positionproximate said front surface.
 19. The invention of claim 18 wherein saidperipheral surface is frustoconical.